many hundreds of millions of years yet to come. The moon, thus regarded, presents to us a most interesting subject of study, because she illustrates, in general respects if not perhaps in details, the condition which our earth will attain in the remote future.
Let us then examine the principal features of the moon, — those which may be regarded as characteristic, which at any rate distinguish her from the earth — and consider how far it is probable that our earth will one day present similar features. We can also inquire how far the moon's present condition may be regarded as that of a dead world, in this sense that she can neither now be, nor (under any conceivable circumstances) hereafter become, once again a habitable world as formerly she presumably was.
There is one very remarkable feature of moon's the moon's motions which is commonly not explained as we are about to explain it, but in a way which would correspond better with the general views indicated in this article, than the interpretation which seems to us preferable. We refer to the circumstance that the moon's rotation on her axis takes place in precisely the same time as her revolution around the earth. This is, in reality, a very strange feature, though it is often dismissed as if there were nothing very remarkable about it. In whatever way the arrangement was brought about, it is absolutely certain that the earth had her share in the work; and again, no matter what explanation or set of explanations we accept, we find most interesting evidence suggested as to the moon's past condition.
According to one account, the moon was originally set spinning at a rate closely corresponding to her present rotation rate, and the earth, having by her attractive power somewhat elongated the moon towards herself, acted on this not perfectly round body in such sort as gradually to coerce its motion of rotation into exact agreement with its motion of revolution. It is known that this would necessarily happen if the original approach to agreement between these motions had been sufficiently close. If we adopted this view, we should find ourselves in presence of the somewhat remarkable fact that the small moon was in the beginning set rotating so slowly that its day a lasted as long as a lunar month. Such a rotation, as the result of some process of systematic evolution, could be readily accepted; but that this motion, which presents no recognizable advantages, and many most manifest inconveniences (for creatures living in the moon), should have been specially communicated to the moon by the creative hand, would not be an acceptable theory, even if we were not forced by overwhelming evidence to throw special creative acts very much farther back (to say the least) than the formation of our moon, or of any part of the solar system.
Another explanation which has been offered runs as follows. When the moon had oceans, the earth must have acted on those oceans in the same way as the moon now acts on the oceans of our earth. In one respect the earth must have acted more energetically, in another less. Being very much (eighty-one times) more massive than the moon, the earth necessarily exerts much more force on the substance than the moon exerts on hers. On the other hand, the relative difference between the pull on the nearest and remotest parts of the globe is less in the case of the earth drawing the waters of the moon (in old times) than in the case of the moon drawing the waters of the earth; for the moon is a much smaller globe than the earth; and this difference is the really effective force in the production of tides. Also it is probable that the moon never had a relatively large ocean surface, as will presently be shown, and small seas (probably disconnected) could not be swept by a great tide-wave. Still we may suppose that there was once a tidal wave, greater or less, sweeping athwart the lunar seas much in the manner of our own tidal wave. Now, our tidal wave is beyond doubt slowly checking the earth's motion of rotation, for the wave travels so as to meet the motion of rotation, which therefore to some slight degree it opposes. This will go on, until at length the rotation has been so reduced that the tidal wave no longer affects it; or, in other words, until the earth's period of rotation corresponds with the period of the tidal wave, viz., with the lunar month. Hundreds of millions of years will pass before that happens; but then we have seen that the moon may fairly be regarded as illus-
- In one sense the moon pulls the earth just as strongly as the earth pulls the moon, for gravity is not a force which one body exerts on another solely, but a mutual force. But what mathematicians call the moving force exerted by the earth on the moon is eighty-one times greater than the corresponding force exerted by the moon on the earth; for the mutual attraction between these bodies has in the former case to move the moon, whereas in the latter it has to move the much larger mass of the earth.